The long term objective of this proposal is to develop a more comprehensive understanding of neonatal vitamin D utilization. Our more immediate objective, however, is to define the antirachitic sterol requirements of preterm and term infants by studying vitamin-D metaboliters and calcium balance responses to various doses of vitamin D, 25-OH-D and 1,25-(OH)2-D. Specifically, we propose to: 1) determine the levels of vitamin D and its metabolites present in mothers' plasma and milk, and correlate these levels with the level of metabolites found in the blood of the nursing infant; 2) determine the plasma levels of vitamin D and its metabolities in newborn infants of varying gestational age and birth weight while these infants are being fed differing vitamin D dietary regimens; 3) make serial measurements of other circulating components that reflect dynamic mineral metabolic status and calcium homeostasis, including parathyroid hormone, calcitonin, total calcium, phosphorus, and magnesium, alkaline phosphatase and ionized calcium; 4) measure calcium and phosphorus absorption and retention using formal balance studies in infants receiving different dietary vitamin D regimens; and 5) monitor neonatal bone growth with differing vitamin D diets by measuring the bone mineral content via direct photon absorptiometry. This proposed study should provide insight into the obscure areas of neonatal vitamin D metabolism and dietary requirements. The data will be of importance in expanding the understanding of the maternal role in supplying antirachitic sterols in milk to the newborn infant. Further, the data will provide information regarding the amount and type of vitamin D metabolities required by neonates at varying gestational ages. Finally, on a long-range basis, the data from this study could result in a redefinition of the vitamin D requirement for the low birth weight and/or premature neonate which would, in turn, faciliate the developoment of synthetic formulas, based on human milk composition, with improved nutritional value for the developing newborn.